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Ch4: Distributed Systems Architectures

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Title: Ch4: Distributed Systems Architectures


1
Ch4 Distributed Systems Architectures

2
  • Typically, system with several interconnected
    computers that do not share clock or memory.
  • Motivation tie together multiple independent
    computers over the network to
  • share resources, enhance performance, improve
    reliability/availability, provide expandability.
  • One possible classification of architecture
  • several minicomputers (machines with CPU/user
    lt 1)
  • several hundred / thousand workstations
    (CPU/user 1)
  • Processor pool (CPU/user gt 1)
  • user typically has a dedicated CPU as well.

3
  • What is an OS
  • An interface between hardware and user processes
    that provides an abstraction of the machine and
    manages its resources.
  • What is a Distributed OS
  • the same, except for distributed systems
  • aims at transparency of distribution and presents
    a virtual uniprocessor to the user.
  • according to some, the holy grail is to create a
    metacomputer and a Problem Solving
    Environment
  • The user sees a single machine that automagically
    provides enough resources to do the task. The
    task can range from simple hello world programs
    to complex calculations.

4
Issues in Distributed OS
  • Global state is not known
  • due to lack of shared memory and clock,
    unreliable message transmission.
  • Need decentralized controls
  • temporal order of events ?
  • Naming
  • how should an object be identified ?
  • Transparent ? Translucent ? Explicit ?
  • URIs, URNs, URLs
  • what if the object is replicated
  • Scalability
  • system should continue to work efficiently as
    resources are added
  • consider a system that resolves IP addresses
    using broadcast

5
  • Compatibility / Interoperability
  • binary level, execution level or protocol level
  • homogeneous vs heterogeneous systems
  • Process Synchronization
  • mutual exclusion problem w/o shared memory
  • Resource Management
  • how do you get data to the location of the
    computation
  • distributed filesystem ? distributed shared
    memory ?
  • how do you migrate a computation (remote
    evaluation)
  • RPC ? Client-Server ?
  • how do you migrate running processes (code on
    demand, mobile objects/agents)
  • Security
  • authentication and authorization in a distributed
    system

6
Structure of Distributed OS
  • Monolithic kernel
  • a (large) single entity that provides all the
    services of the distributed OS
  • may not be a good idea since computer
    configurations will vary
  • do you need to load disk drivers on a diskless
    client ?
  • Collective kernel
  • base OS functionality is in a relatively small
    microkernel. All other OS services are processes
    that run on top.
  • Microkernel will run on all machines
  • OO approach
  • same as collective kernel, but OS services are
    implemented as objects
  • can provide a more structured approach than
    collective kernel

7
Communication
  • Review Section 4.6 yourself.
  • Message Passing Model
  • send and receive type primitives
  • can be blocking(reliable, unreliable) or
    non-blocking
  • use of buffers
  • synchronous vs asynchronous

8
Millennium Project
  • Work at Microsoft Research(paper by Bolosky et
    al.)
  • Features
  • seamless distribution, worldwide scalability,
    fault tolerance, self-tuning, self configuring,
    secure, resource control
  • Principles
  • aggressive abstraction, storage irrelevance,
    location irrelevance, JIT binding, introspection.
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